Previous work from this laboratory established that plasma metanephrines, the O-methylated metabolites of catecholamines, provide a superior diagnostic marker of pheochromocytoma than previously available tests. The superiority of this test has now been established to result from production of considerable amounts of metanephrines within tumors, this secondary to the presence of large amounts of the membrane-bound form of catechol- O-methyltrasferase within chromaffin cells. Some tumors are "silent" and do not secrete catecholamines in amounts sufficient to cause the classic symptoms of a pheochromocytoma or a positive plasma or urinary catecholamine test result. Other tumors secrete catecholamines episodically. However, even when not actively secreting catecholamines the tumors are actively metabolizing catecholamines to metanephrines. Ongoing work with collaborators throughout the US and in Sweden, Germany and the Netherlands involves expanding the patient data base by examining patient groups where the differential diagnosis of pheochromocytoma is particularly troublesome (e.g. incidentalomas, renal failure, hypernoradrenergic hypertension, panic disorder, familial pheochromocytoma). A long-term goal is to develop guidelines for the use of appropriate diagnostic strategies to identify within these groups the occasional patient with pheochromocytoma. A study of patients with a familial predisposition to develop pheochromocytomas, carried out in 21 patients with von Hippel Lindau (VHL) disease and 9 patients with multiple endocrine neoplasia type II (MEN-II), has established that plasma free metanephrines are particularly useful for screening pheochromocytomas in these at risk patient groups. Biochemical testing indicated a sensitivity of 97% for plasma metanephrines, 73% for plasma catecholamines, 70% for urinary catecholamines, 66% for urinary metanephrines and 51% for urinary vanillylmandelic acid. The single VHL patient with normal plasma metanephrines had a very small adrenal tumor (<1 cm) and normal values for all biochemical tests. Another 4 VHL patients had normal values for all biochemical tests apart from plasma normetanephrine. In 2 of these patients, studied over 4-5 years before removal of tumors, plasma normetanephrine was elevated at all times and climbed as tumors developed, whereas all other biochemical tests were normal and showed no trend with time. This study also indicated completely different tumor phenotypes in VHL and MEN-II patients. All MEN-2 patients had elevations in plasma metanephrine, whereas VHL patients typically showed only elevated plasma concentrations of normetanephrine. Studies are currently underway to characterize these phenotypic differences at the cell biological level (adrenergic versus noradrenergic, presence of glucagon receptors and norepinephrine transporters) and relate any observed differences to clinical observations about these patient groups.